A number of electro-optic and acousto-optic switches and deflectors have been developed in both bulk and thin-film forms. Many of these have rather high efficiencies, but most are characterized by having intrinsically small angular deflection ranges. In this introductory section, we report initial experimental results on a two-position electro-optic switch which can be fabricated both in a bulk configuration and in thin-film configurations suitable for integrated optics applications. The switch has the advantage of being able to function at any, arbitrarily large, predetermined angle. In both the bulk and thin-film forms, the switches can be cascaded into a binary array.
The wide-angle switch is based on the high efficiency and high angular selectivity of thick phase gratings. Using holographic techniques it is not difficult to construct such gratings with an angular selectivity of a few millidegrees about the Bragg angle. Forming the gratings in an electro-optic material via the "optical damage" effect makes it possible to change the average index of refraction in the grating region by the application of an electric field. This index change, in turn, causes a change in the Bragg angle of the grating which can be sufficient to take the fixed incident beam out of (or not) Bragg incidence. The resulting deflection of the beam can be made as large as desired by altering the grating spacing.
An analysis of the effect described above, based on the coupled-wave theory of Kogelnik and including the additional effects due to the piezoelectric effect, has been carried out and is presented elsewhere. For out present purposes we need only note that the Bragg angle is determined from EQU cos (.phi.- .theta..sub.B) = .lambda..sub.O /2n.LAMBDA., (1)
where .phi. is the slant angle and specifies the orientation of the grating relative to the physical boundaries of the crystal, .theta..sub.B is the Bragg angle measured inside the crystal, .lambda..sub.O is the free-space wavelength of the light, n is the refractive index, and .LAMBDA. is the grating period. In the presence of an electric field, these quantities change according to ##EQU1## where the change in grating period, dln.LAMBDA./dE, and the rotation of the grating, d.phi./dE, are due to the piezoelectric effect, and the change in refractive index, dln(n)/dE, is due to the electrooptic effect. Eq. 2 relates to Bragg angle changes to applied field. Since the incident beam is fixed in direction, this change in Bragg angle is equivalent to a misalignment of the incident beam, and the diffraction efficiency of the grating decreases (increases) if the initial alignment was on (off) Bragg incidence. An estimate of the field required to observe the effect in bulk in LiNbO.sub.3 has been made using Kogelnik's expression for the diffraction efficiency and ignoring the piezoelectric effect. A field of the order of 10.sup.4 V/cm should suffice for the grating setup of FIG. 4. In the more common arrangement for writing gratings, where the grating planes are perpendicular to the entrance face of the crystal, the electro-optic effect will not produce any deflection and only the piezoelectric effect will contribute.
An experimental arrangement used to demonstrate the switch effect is shown in FIG. 4. Using the indicated geometry, a 6200-line/mm phase grating was written with the grating vector inclined at an angle of 50.degree. with respect to the normal to the sample surface. The sample used was a 25 .times. 25 .times. 3-mm LiNbO.sub.3 crystal grown from a congruent melt. No attempt was made to optimize the grating efficiency or fix the grating after it was written. The grating was both written and probed with the 4880-A line of an argon-ion laser. The write-only beam is incident at an angle of 24.degree. to the normal to the sample face. The read-and-write beam is normally incident on the sample edge. The argon ion writing beams were approximately 2 mm in diameter at the crystal surfaces. The measured angular half-power points of the resultant grating efficiency curve corresponded to an angular acceptance of about 14 mrad.
After writing the grating, the writing shutter was closed and the intensity of the diffracted portion of the read-and write beam was monitored. Application of a 2000-V 100-.mu.sec pulse across the 2-mm electrode gap resulted in a 35% reduction of the intensity of the diffracted beam. Adjustment of the angle of incidence of the reading beam so that it was slightly off the Bragg angle with the field off allowed the voltage pulse to cause an increase in the diffracted beam intensity. Thus both normally on and normally off switching arrangements can be used. The oscilloscope 51 trace of the photomultiplier output shows (upper trace) 55 response of the wide-angle switch element to a 1000-V/mm 100-.mu. sec pulse. The lower trace 56 was taken with the oscilloscope input shorted out, providing a base line for reference. The modulation of the diffracted beam was about 35%.